Known lighting circuits for a discharge lamp such as a metal halide lamp include a DC power supply circuit based on a DC-DC converter configuration, and a configuration including a DC-AC converter circuit and a starter circuit. For example, a DC input voltage from a battery is converted to a desired voltage in a DC power supply circuit, and the resulting DC voltage is then converted to an AC output by a subsequent DC-AC converter circuit. A high-voltage signal for starting is multiplexed on the AC output, and the resulting multiplexed signal is supplied to a discharge lamp (see, e.g., JP-A-7-142182).
In a configuration which converts a voltage at two stages (DC-DC voltage conversion and DC-AC voltage conversion), a larger circuit scale is unsuitable for a reduction size, so that a discharge lamp is supplied with an output which is boosted through a single-stage voltage conversion performed in a DC-AC converter circuit (see, e.g., JP-A-7-169584).
Then, a driving control (for controlling the frequency of a switching element) associated with the DC-AC converter circuit is conducted to control a non-load output voltage (hereinafter called “OCV) before the discharge lamp is lit (during extinction), to bring the discharge light toward a steady lighting state, while reducing transient power applied thereto, after the discharge light is turned on by applying a starting signal thereto.
Such conventional lighting circuits may be susceptible to extinction as a result of reduced maximally available power caused by an excessively reduced input voltage from the DC power supply. To prevent such a problem, complicated control components may be required.
For example, measures are required for the extinction of a discharge lamp used as a car illumination light source as a result of a shortage of the power supplied to the discharge lamp when the battery voltage becomes lower. Specifically, while the power supplied to the discharge lamp may be interrupted when the battery voltage is reduced to a predetermined threshold or lower, it is desirable to maintain the discharge lamp in the lighting state, in order to ensure the safety in a night run, by controlling the power supplied to the discharge lamp as long as the discharge lamp can be kept lit.
In a configuration which controls output power for a lighting circuit by controlling a switching frequency in a converter circuit, the power is controlled by setting the switching frequency (or a lighting frequency) to a predetermined frequency or higher such that the discharge light is supplied with substantially constant power in a normal stable lighting state. Here, the “predetermined frequency” means a driving frequency at which the output voltage or output power is maximized when the discharge lamp is lit (this frequency is labeled “f2”).
As the capabilities of the lighting circuit are degraded by a reduced DC input voltage to output a lower voltage, the frequency is controlled to provide constant power by bringing the lighting frequency closer to the aforementioned f2.
However, if the DC input voltage is suddenly reduced for some reason, the discharge lamp cannot be kept lit unless sufficient power is outputted to the discharge lamp, even if the lighting frequency is set to f2, resulting in a higher probability of a failure in lighting (i.e., measures must be taken for compensating for a shortage of power).
It is, therefore, desirable to ensure that the discharge lamp is kept lit even when a DC input voltage is reduced without requiring a complicated circuit configuration or control method.